THE EVOLUTIONARY STATUS OF UX ORIONIS-TYPE STARS

We present measurements of 1.3 mm continuum emission from a sample of UX Ori stars (UXOrs). The UXOrs are pre-main-sequence stars, typically of intermediate mass, and they are distinguished from other pre-main- sequence stars by their large photometric and polarimetric variations that are thought to be due to variable extinction by circumstellar dus t. Transient optical and UV redshifted absorption lines are a second d efining characteristic, and have been interpreted in terms of the disr uption of infalling cometary bodies. Our new millimeter fluxes are use d to derive masses of circumstellar dust M-CSD. We combine these measu rements with data from the literature to examine a sample of 30 pre-ma in-sequence systems with spectral types F0-B9, i.e., Herbig Ae stars, about half of which display UXOr characteristics. We find no systemati c differences in M-CSD between the two groups of stars. Moreover, no s tatistically significant correlation between M-CSD and stellar age is found, and the amplitude of photometric variability appears to be inde pendent of age. We propose that UXOr phenomena do not characterize a m ore evolved environment. They are probably common to the majority of s tars in our sample, but are observed only when the line of sight is cl ose to the equatorial plane of an aspheric circumstellar nebula.